As is known, using electromechanical microstructures of semiconductor material, the manufacture of which utilizes microelectronics techniques, has recently been proposed for producing accelerometers and gyroscopes. These silicon micro-machining techniques make it possible to produce different types of angular velocity and acceleration sensors. In particular, at the present lime prototypes operating according to the piezoelectric, piezoresistive, capacitive, threshold, resonant and tunnel effect principles have been proposed.
Reference will be made below to an accelerometric sensor of differential capacitive type, i.e. one in which acceleration induces the movement of a seismic mass which constitutes the electrode common to two coupled capacitors by varying the two capacitances in opposite directions. This effect is known as differential variation of capacitance.
Historically, integrated micro-structures have been manufactured by preferably using the “bulk micro-machining” technique in which a wafer of single-crystal silicon is machined on both faces. This technique is, however, incompatible with the process steps for producing components of a circuit which processes a signal picked up by a sensitive element, as required at present.
It has been proposed to use the technique of “surface micro-machining” in which the sensitive element is made of multi-crystal silicon and suspended structures are formed by depositing and successively removing sacrificial layers. This technique is compatible with the current integrated circuit manufacturing processes and is therefore preferred at present. The integrated micro-structures produced with this technique are, however, relatively insensitive to acceleration and angular velocity. In fact, having a mass of the order of a few tenths of a microgram, they suffer the effects of thermodynamic noise caused by the Brownian movement of the particles of the fluid in which they are immersed (see, for example, the article by T. B. Gabrielson entitled “Mechanical-Thermal Noise in Micromachined Acoustic and Vibration Sensors”, IEEE Transactions on Electron Devices, vol. 40, No. 5, May 1993). The upper limit to the mass obtainable with these structures is imposed by genuinely technological reasons; the deposition of very thick films involves extremely long wafer machining times and renders the surface of the wafer unsuitable for the successive operations such as lapping the wafers.
A technique for machining the epitaxial layer (epitaxial micro-machining) is also known, which produces micro-structures with inertial masses that are higher and hence more sensitive, but not yet at a sufficient value for practical applications.